Method and apparatus for testing telephone sound quality

ABSTRACT

A method and apparatus for testing telephone sets to determine their sound quality comprising a system for repeatedly applying a sequence of pulse tones to a test telephone where the respective tone frequencies are different and within the normal human voice range.

RELATED APPLICATIONS

This application incorporates and claims the benefit of U.S. ProvisionalApplication 60/369,540 filed Apr. 1, 2002.

FIELD OF THE INVENTION

This invention relates generally to telephones and more particularly toa method and apparatus for testing telephone sets to determine theirsound quality.

BACKGROUND OF THE INVENTION

Telephone sets, i.e., base unit and handset, whether corded, cordless,or speakerphone, typically employ Automatic Gain Control (AGC) or othernonlinear circuits to suppress telephone line noise, static, and/oracoustic feedback. Such circuits are intended to eliminate or reducenoise that would otherwise be heard by a user at the handset speakerduring a telephone communication. However, in some designs, suchcircuits can operate excessively with the result that the user is likelyto hear a “choppy” sound quality. That is, the user may find that a farend talker with a quiet voice is difficult or impossible to understandbecause too many of the talker's syllables are suppressed or cutoff,resulting in the “choppy” sound.

Additionally potential sources of poor telephone sound quality include:

-   -   Signal overload with resulting distortion    -   Signal underload with resulting noise    -   Insufficient digital quantization resolution and/or range    -   Low digital sampling rate    -   Excessive low pass and/or dynamic filtering    -   Excessive cordless telephone RF carrier dropout

Currently used standard acoustic tests frequently produce results thatdo not adequately correlate to the human perception of a telephone'ssound quality. For instance, the measured audio frequency response of atelephone unit A can fail to meet specified limits and yet be rankedmuch higher by a typical user than a telephone unit B whose measuredaudio frequency response complies with the specified limits.

The present invention is directed to an improved method and apparatusintended to determine the subjective sound quality of a telephone unit.

SUMMARY OF THE INVENTION

More particularly, the present invention is directed to a system whichrepeatedly applies a sequence of pulse tones of different frequency andamplitude to a telephone set under test (i.e., “test telephone”) todetermine its sound quality.

In accordance with a preferred embodiment, each pulse tone sequence(PTS) is comprised of three tones (sine waves) which are electronicallyinjected into the test telephone to simulate a talker's voice. Moreparticularly, the three tones are preferably selected to havefrequencies in the middle of the human voice range (e.g., 800 Hz, 1000Hz, 1200 Hz) that most users can readily hear and distinguish.

Additionally, in accordance with the preferred embodiment, the multipletones in each sequence have amplitudes selected to simulate a moderatelyquiet talker that most listeners can readily hear in a normal roomenvironment.; In the preferred three tone sequence, the first and lasttones are preferably selected to have amplitudes noticeably lower thanthe middle tone to simulate the lower amplitude syllables in humanspeech that are often lost, i.e., chopped off, in telephonecommunications.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a circuit diagram of an exemplary arrangement for testing atelephone set in accordance with the present invention;

FIG. 2A depicts an oscilloscope display produced by a reference cordedtelephone excited by a pulse tone sequence in accordance with thepresent invention and FIG. 2B comprises a table describing test resultsassociated with the display of FIG. 2A.

FIG. 3A depicts an oscilloscope display produced by an exemplarycordless test telephone excited by the same pulse tone sequence as usedin FIG. 2A and FIG. 3B comprises a table describing test resultsassociated with the display of FIG. 3A;

DETAILED DESCRIPTION

Attention is initially directed to FIG. 1 which depicts a preferredarrangement for testing a telephone set 10 in accordance with thepresent invention. The telephone set 10 is comprised of a base unit 12which communicates with a cordless handset 14 via an RF link 16.

In order to perform the test to be described hereinafter, a signalgenerator 20 and battery feed circuit 22 are connected to electricallyinject a pulse tone sequence into base unit 12 via a simulatednon-loaded cable 23 for transmission to handset 14. The handset 14typically includes a transmit, or microphone, portion 26 and a receive,or speaker, portion 28. As illustrated, the transmit portion 26 ispreferably sealed with a sound attenuating cover 30.

The receive portion 28 is acoustically coupled to an artificial ear 32which responds to applied acoustic energy to drive amplifier 34. Theoutput of amplifier 34 is applied to an oscilloscope 36 for displayingthe characteristics of the acoustic energy produced by the handsetreceive portion 28.

In order to test the sound quality of the telephone set 10 in accordancewith the invention, a sequence of multiple pulse tones produced bysignal generator 20 is repeatedly applied to base unit 12. The pulsetone sequence (PTS) is selected to simulate a moderately quiet talker ina normal environment. In accordance with a preferred embodiment of theinvention, the selected pulse tone sequence is comprised of threedistinct successively generated sine waves of different frequencieswithin the human voice range. The following specification describes apreferred embodiment of the present invention.

Preferred Test Requirements

The telephone set shall be excited by a selected three tone sequence todetermine whether it clearly reproduces all three tones of the sequence.The telephone set shall be considered as passing the test if either theObjective or the Subjective PTS Test requirement is met:

-   -   (a) Objective PTS Test—The first and third tones, at 10% of the        amplitude of the middle tone, shall be evident at the        oscilloscope 36. Each of the three tones shall be 200        milliseconds wide.    -   (b) Subjective PTS Test—All three tones shall be clearly heard        without audible suppression and without audible rounding off of        any of the tones.        Standard Method of Measurement    -   (a) The PTS test shall be performed using the test circuitry        shown in FIG. 1.    -   (b) The signal generator 20 shall produce the pulse tone        sequence (i.e., “test signal”) repeatedly. Each cycle of the        test signal shall include a triple-tone sequence, followed by        400 milliseconds of silence, for a total cycles duration of 1000        milliseconds (1 second). The sine wave frequencies, amplitudes,        and durations shall be generated in the sequence listed in the        following Table 1.

TABLE 1 PTS Test Signal Frequencies, Amplitudes, and Durations Sequence1 2 3 Frequency 800 Hz 1000 Hz 1200 Hz Amplitude  5 mV peak  50 mV peak  5 mV peak Duration 200 milliseconds  200 milliseconds  200milliseconds

-   -   (c) The measurement shall be made for 0 km of 26 AWG non-loaded        cable as shown in FIG. 1.    -   (d) If the telephone handset has a Receive volume control, the        control shall be set to the nominal gain level.    -   (e) The handset should be in an ambient noise environment no        greater than 40 dB(A) Sound Pressure Level referenced to 20        micropascals, and the handset transmitter should be covered, as        by cover 30.    -   (f) To evaluate the Objective Receive Sound Quality, an        oscilloscope 36 or equivalent instrument is connected to the        output of amplifier 34 and the instrument time base is adjusted        to display two complete cycles of the PTS test signal. The        display is adjusted so that the amplitude of the middle (1000        Hz) tone is near full scale. The relative amplitudes of the        first (800 Hz) and third (1200 Hz) tones are observed and        compared to the middle tone, as well as their pulse widths.    -   As an example, the display of a reference corded telephone which        passes this test is shown in FIG. 2A. Note that the first and        third tones are evident at 10% of the amplitude of the middle        tone, and each tone is about 200 milliseconds wide. The display        of a cordless telephone which fails this test is shown in FIG.        3A; the first and third tones are not evident, and the second        tone is noticeably less than 200 milliseconds wide.    -   (g) To evaluate the Subjective Receive Sound Quality, a        technician shall listen to the handset receiver for all three        tones of the PTS test signal. A reference corded telephone        should preferably be used for comparison purposes; e.g., the        reference corded telephone of FIG. 2A which produces the        subjective results summarized in the Table of FIG. 2B where all        three tones are clearly heard by the testing technician. The        performance of an exemplary cordless telephone which fails this        subjective test is summarized in the Table of FIG. 3B where the        first and third tones are not clearly heard, sounding suppressed        or rounded off.        Expanded Measurements

To help determine the level of “choppy” sound in a telephone, the PTStest may be performed at amplitudes above and below the standard (5, 50,5 mV peak) level; the following Table 2 lists the recommended PTSlevels. All other parameters (e.g., sequence, frequency, duration, andsilence) and procedures are identical to those previously discussed.

TABLE 2 Expanded PTS Test Amplitudes PTS Level 800 Hz 1000 Hz 1200 HZVery High 20 mV peak 200 mV peak 20 mV peak High 10 mV peak 100 mV peak10 mV peak Standard  5 mV peak  50 mV peak  5 mV peak Low  2 mV peak  20mV peak  2 mV peak Very Low  1 mV peak  10 mV peak  1 mV peak

If a telephone set fails to reproduce all three tones of the PTS testsignal at the Very High Level, the unit has a very high level of“choppy” sound and thus has very poor Receive Sound Quality. If atelephone reproduces all three tones of the PTS test signals at he VeryLow Level, the unit has a very low level of “choppy” sound and thus hasvery good Receive Sound Quality.

As an example, a reference corded telephone which passes all of theexpanded measurements is summarized in the following Table 3;

TABLE 3 Expanded Objective/Subjective PTS Test- Reference CordedTelephone Results PTS Level 800 Hz 1000 Hz 1200 Hz Pass/Fail Very HighYes Yes Yes Pass (20, 200, 20 mV peak) High Yes Yes Yes Pass (10, 100,10 mV peak) Standard Yes Yes Yes Pass (5, 50, 5 mV peak) Low Yes Yes YesPass (2, 20, 2 mV peak) Very Low Yes Yes Yes Pass (1, 10, 1 mV peak)Key: Yes = tone clearly visible or audible No = tone suppressed orrounded offNote in Table 3 that all three tones are clearly reproduced at all PTSlevels. A cordless telephone which fails most of the expandedmeasurements is summarized in Table 4.

TABLE 4 Expanded Objective/Subjective PTS Test-Cordless TelephoneResults PTS Level 800 Hz 1000 Hz 1200 Hz Pass/Fail Very High Yes Yes YesPass (20, 200, 20 mV peak) High No Yes Yes Fail (10, 100, 10 mV peak)Standard No Yes¹ No Fail (5, 50, 5 mV peak) Low No Yes¹ No Fail (2, 20,2 mV peak) Very Low No No No Fail (1, 10, 1 mV peak) Key: Yes = toneclearly visible or audible No = tone suppressed or rounded off Note:¹Objectively, the tone was rounded off (“No”), but Subjectively, thetone was clearly audible (“Yes”) for an overall “Yes”.Note that at the High (10, 100, 10 mV peak) level, the first (800 Hz)tone is not clearly reproduced, so the result is a “Fail” for the HighPTS level.

Additionally, the PTS test measurement may be repeated with longer loops(e.g., 2.7 and 4.6 km of 26 AWG non-loaded cable) to determine the levelof “choppy” sound with a range of loop conditions.

From the foregoing, it should now be appreciated that a method andapparatus have been described for testing telephone sets to determinetheir sound quality. Although only a limited number of specificembodiments have been described, it is recognized that variousmodification and equivalents will occur to those skilled in the artfalling within the spirit and intended scope of the invention.

1. A method of evaluating the sound quality of a telephone set comprisedof a base unit for providing an electric drive signal and a handsetresponsive to the drive signal for producing an acoustic output, themethod comprising: generating an electric test signal characterized by afirst pulse tone having a frequency F1 at amplitude A1, a second pulsetone having a frequency F2 at amplitude A2, and a third pulse tonehaving a frequency F3 at amplitude A3; injecting the test signal intothe base unit to transmit a corresponding drive signal to the handset;and evaluating the sound quality of the telephone set by determiningwhether the acoustic output produced by the handset includes threediscernibly distinct tones.
 2. The method of claim 1 wherein thegenerating step includes generating a test signal comprised ofsuccessive cycles wherein each cycle is characterized by the successivegeneration of the first, second, and third pulse tones during first,second, and third pulse intervals respectively.
 3. The method of claim 1wherein said the pulse tone frequencies are related as F3>F2>F1.
 4. Themethod of claim 1 wherein the pulse tone frequencies F1, F2, F3substantially equal 800 Hz, 1000 Hz, 1200 Hz, respectively.
 5. Themethod of claim 1 wherein at least two of the pulse tone amplitudes areunequal.
 6. The method of claim 1 wherein the ratio between theamplitudes of two of the pulse tones is greater than five.
 7. The methodof claim 1 wherein the ratio of the pulse tone amplitudes A1, A2, A3 isdefined approximately by 1:10:1.
 8. The method of claim 2 wherein eachpulse tone interval has a duration of about 200 milliseconds.
 9. Themethod of claim 2 wherein each cycle farther includes a no-pulseinterval.
 10. The method of claim 9 wherein each pulse tone interval hasa duration of about 200 milliseconds and the no-pulse interval has aduration of about 400 milliseconds.
 11. The method of claim 1 whereinevaluating the sound quality includes a person listening to the acousticoutput produced by the handset to reach a subjective judgment.
 12. Themethod of claim 1 wherein evaluating the sound quality includes usingthe acoustic output to produce a graphical display.
 13. The method ofclaim 1 wherein the drive signal is transmitted to the handset via awire.
 14. The method of claim 1 wherein the drive signal is transmittedto the handset via a wireless RF link.
 15. An apparatus for testingsound quality of a telephone set comprised of a base unit for providingan electric drive signal to a handset for producing an acoustic output,the apparatus comprising: signal generator means for supplying a testsignal to the base unit characterized by a first pulse tone having afrequency F1 at amplitude A1, a second pulse tone having a frequency F2at amplitude A2, and a third pulse tone having a frequency F3 atamplitude A3; the test signal being comprised of successive cycleswherein each cycle is characterized by first, second, and third pulseintervals respectively containing the first, second, and third pulsetones; and means responsive to the acoustic output produced by thehandset for evaluating the sound quality of the telephone set bydetermining whether the acoustic output produced by the handset includesthree discernibly distinct tones.
 16. The apparatus of claim 15 whereinthe test signal frequency F3 is greater than frequency F2 which isgreater that frequency F1.
 17. The apparatus of claim 15 wherein thepulse tone amplitudes A1, A2, A3, are not all equal.
 18. The apparatusof claim 15 wherein one of the pulse tone amplitudes is greater thanfive times another of the pulse tone amplitudes.
 19. The apparatus ofclaim 15 wherein the means responsive to the acoustic output includes adisplay device for graphically representing the acoustic output.
 20. Amethod comprising: injecting an electric test signal into a base unit ofa telephone to transmit a corresponding drive signal to a handset of thetelephone, the test signal comprising first, second, and thirdsuccessive pulse tones, the first pulse tone having a frequency F1 andan amplitude A1, the second pulse tone having a frequency F2 greaterthan F1 and an amplitude A2, and the third pulse tone having a frequencyF3 greater than F2 and an amplitude A3, wherein A2 is at least fivetimes higher than each of A1 and A3; and evaluating the sound quality ofthe telephone set by determining whether the acoustic output produced bythe handset includes three discernibly distinct tones.